Polyoxymethylenes have been produced by a method of subjecting trioxane to bulk homopolymerization or subjecting trioxane with ethylene oxide or trioxane with a cyclic formal such as dioxolane, trioxepane or the like, to bulk copolymerization in the presence of a cationically active catalyst.
In conventional polyoxymethylene production, a reactor of two shafts self-cleaning type, such as disclosed in JP-A-51-84890 has been used mainly. In this reactor of two shafts self-cleaning type, the discharging port for reaction mixture is provided at the bottom of the reactor and consequently the polyoxymethylene formed as an intended product is recovered downward. This reactor, however, has had a problem in that the short residence time of reaction mixture gives a low conversion. Hence, a number of proposals have been made in order to make the residence time longer and the conversion higher.
For example, JP-A-56-59824 proposes a process wherein, in each two adjacent paddles fixed to a rotary shaft of a reactor, the angle of one (back) paddle is advanced by a given amount from the angle of the other (precedent) paddle to control the transfer speed of reaction mixture to increase the filling ratio of the reaction mixture and prolong the residence time of the reaction mixture. Also, JP-A-61-238812 proposes a process wherein a reactor is lifted at the discharging port side so that the reactor is tilted as a whole by 1-10.degree. from the horizontal state to prolong the residence time of the reaction mixture.
However, in the above process wherein the angle of one paddle is advanced from the angle of the other paddle to prolong the residence time, when the filling ratio is too high, the torque required for stirring increases sharply and surpasses the endurance limit of reactor (torque-out), making the continued reactor operation impossible. In the above process wherein a reactor is tilted slightly as a whole, both a very high conversion and very stable operation can be achieved; however, L/D (L: length of reactor, D: inside diameter of reactor) must be large in order to make sufficiently long the residence time, which requires a large reactor not suitable for industrial use.
The object of the present invention is to provide a process for producing a polyoxymethylene, wherein the residence time of reaction mixture in a reactor is long and thereby the conversion of trioxane can be increased.